A steering apparatus of an automobile is configured as shown in FIG. 12. The motion of a steering wheel 1 operated by a driver is transmitted to an input shaft 6 of a steering gear unit 5 through a steering shaft 2, a universal joint 3a, an intermediate shaft 4 and another universal joint 3b. A pair of tie rods 7, 7 is pushed or pulled by a rack and pinion mechanism installed in the steering gear unit 5, so that an appropriate steering angle is applied to a pair of left and right steering wheels, in conformity to an operation amount of the steering wheel 1.
FIG. 13 illustrates an example of the intermediate shaft 4 that is mounted to the steering apparatus as described above. In this example, the intermediate shaft 4 is configured to expand and contract so as to prevent the steering wheel 1 from being pushed towards the driver upon a collision accident. The intermediate shaft 4 includes an inner shaft 9 having a male spline part 8 provided on an outer periphery of a tip portion (a left end portion in FIG. 13) thereof, and a circular tube-shaped outer tube 11 having a female spline part 10 formed on an inner periphery thereof to which the male spline part 8 can be inserted. The male spline part 8 and the female spline part 10 are spline-engaged with each other, so that the inner shaft 9 and the outer tube 11 are combined to be expandable and contractible. Also, base end portions of yokes 12a, 12b configuring the universal joints 3a, 3b are welded and fixed to base end portions of the inner shaft 9 and the outer tube 11, respectively.
FIGS. 14 and 15 illustrate a first example of a known universal joint, which can be used as the universal joints 3a, 3b and is illustrated in Patent Documents 1 and 2. In the meantime, the structure shown in FIGS. 14 and 15 is a so-called vibration preventing joint configured to prevent vibration transmission. However, a universal joint, which is a subject of the present invention, is not necessarily required to have a vibration preventing structure. Therefore, in the below, the vibration preventing structure is omitted and a main body structure of the universal joint 3 is described.
The universal joint 3 is configured by coupling a pair of bifurcated yokes 12a, 12b made of a metal material having sufficient stiffness through a cross shaft 13 made of hard metal such as alloy steel such as bearing steel so that torque can be transmitted. Both yokes 12a, 12b have base parts 14, 14, respectively, and each of yokes 12a, 12b has a pair of coupling arm parts 15, 15. Both base parts 14, 14 are configured to support and fix the base end portion of the inner shaft 9 or outer tube 11 (which is a rotary shaft) (or a front end portion of the steering shaft 2 or a rear end portion of the input shaft 6, refer to FIG. 12) so that the torque can be transmitted. Tips of the coupling arm parts 15, 15 are respectively formed with circular holes 16, 16 to be concentric each other, for each of the yokes 12a, 12b. Cylindrical bearing cups 17, 17 made of a plate material of hard metal such as bearing steel and case-hardening steel to have a bottom are fastened and fitted with openings thereof facing each other to the respective circular holes 16, 16, so that they are internally fitted. The cross shaft 13 has such a shape that intermediate parts of a pair of column parts are orthogonal to each other, and has four shaft parts 18, 18 each of which has a cylindrical shape. That is, base end portions of the respective shaft parts 18, 18 are coupled and fixed to four positions (at a state where center axes of the adjacent shaft parts 18, 18 are orthogonal to each other) equally spaced in a circumferential direction of a coupling base part 19 provided at a center part of the cross shaft. The center axes of the respective shaft parts 18, 18 exist on the same plane.
The shaft parts 18, 18 are inserted from axial intermediate parts to tip portions thereof in the respective bearing cups 17, 17. A plurality of needles 20, 20 each of which is a rolling body are arranged between inner peripheries of the respective bearing cups 17, 17 and outer peripheries of the tip portions of the respective shaft parts 18, 18, so that radial bearings 21, 21 are configured and both the yokes 12a, 12b can be pivotally displaced relative to the cross shaft 13 by small force. With this configuration, even when the center axes of both the yokes 12a, 12b do not coincide with each other, the rotational force can be transmitted between both the yokes 12a, 12b with transmission loss being suppressed.
According to the universal joint 3 as described above, center parts of the respective shaft parts 18, 18 are formed with bottomed insertion holes 22, 22 with being opened towards end surfaces of the respective shaft parts 18, 18 in axial directions of the respective shaft parts 18, 18. In the respective insertion holes 22, 22, pins 23, 23 made of a synthetic resin are inserted, respectively. The respective pins 23, 23 are supported between the respective bearing cups 17, 17 and the respective shaft parts 18, 18 to prevent the respective bearing cups 17, 17 from rattling relative to the respective shaft parts 18, 18 and both the yokes 12a, 12b from rattling relative to the cross shaft 13 and to prevent distances between the opening end portions of the respective bearing cups 17, 17 and the coupling base part 19 from being excessively narrowed. That is, in the case of the universal joint 3b, which is mounted at an outside (at a lower side in FIG. 12) of a vehicle interior, of the universal joints 3a, 3b configuring the steering apparatus shown in FIG. 12, seal rings 24, 24 are respectively provided between the base end portions of the respective shaft parts 18, 18 configuring the cross shaft 13 and the openings of the respective bearing cups 17, 17. In this example, the respective pins 23, 23 are provided to prevent the endurance of the respective seal rings 24, 24 from being lowered due to the excessive compression of the respective seal rings 24, 24 and to prevent the sealing characteristics of the respective seal rings 24, 24 from being deteriorated due to the excessive lowering of the compression amounts of the respective seal rings 24, 24.
FIGS. 16, 17A and 17B illustrate a second example of the structure of the known universal joint, which is disclosed in Patent Document 3. In the second example, a thrust piece 25 having a substantially disc shape and made of an elastic synthetic resin is interposed between a bottom inner surface of the bearing cup 17 configuring the radial bearing 21 and an end surface of a shaft part 18a configuring a cross shaft 13a, as shown in FIGS. 17A and 17B. In the second example, the thrust piece 25 is supported between the bottom inner surface of the bearing cup 17 and the end surface of the shaft part 18a, so that the rattling of the yoke 12 relative to the cross shaft 13a can be prevented.
In either structure, the pin 23 or thrust piece 25 is supported between the bearing cup 17 and the shaft parts 18, 18a, so that the rattling of the yokes 12, 12a, 12b relative to the cross shafts 13, 13a can be prevented. However, there is still room for improvement on the function of suppressing the rattling of the pair of yokes relative to the cross shaft while suppressing an increase in the manufacturing cost. That is, in order to suppress the rattling of both the yokes relative to the cross shaft, it may be conceivable of enlarging a fitting margin of the thrust piece to the bearing cup and the shaft part of the cross shaft. However, when the fitting margin is simply enlarged, a rotational resistance (pivotal resistance) of both the yokes relative to the respective shaft parts is increased. Therefore, in order to prevent the increase in the rotational resistance while suppressing the rattling, it is necessary to form the thrust piece (or pin and insertion hole) with high precision and to enhance the assembling precision (an insertion amount of the shaft part into the bearing cup) of the cross shaft and the yoke, which increase the manufacturing cost of the cross shaft universal joint.